Fluid mixing device



Sept. 23, 1969 a. L. HAMMOND 3,468,637

FLUID MIXING DEVICE Filed June 9, 1965 Inventor Gerald L. Hammond yUnited States Patent 3,468,637 FLUID MIXING DEVICE Gerald L. Hammond,Chicago, Ill., assignor, by mesne assignments, to Precision ScientificCompany, Chicago,

111., a corporation of Delaware Filed June 9, 1965, Ser. No. 462,520Int. Cl. G01n 31/00; B011 /00 US. Cl. 23-292 1 Claim The presentinvention relates generally to devices for mixing fluids and, moreparticularly, to an improved fluid mixing device suitable for use inhigh speed colorimetric gas analyzers.

It is a primary object of this invention to provide an improved mixingdevice for rapidly achieving an intimate admixture of two or more fluidswhile maintaining a high flow rate of the fluids being mixed. A relatedobject is to provide such a device which is suitable for use incontacting a liquid colorimetric reagent with a gas sample in a highspeed colorimetric gas analyzer. Thus, it is an object of the inventionto provide such a mixing device which provides a minimum response timefor the colorimetric reagent in such an analyzer.

It is another object of this invention to provide an improved fluidmixing device of the foregoing type which provides a .minimum volumeholdup of the fluids being mixed. In this connection, it is an object toprovide such a device which maintains a continuous and rapid fluid flowduring the mixing operation.

It is a further object of the invention to provide an improved fluidmixing device of the type described above which is extremely compact soas to be compatible with small portable gas analyzing instruments.

Still another object of this invention is to provide an improved fluidmixing device of the above type which is simple and economical tomanufacture and maintain. A related object is to provide such a devicewhich is completely enclosed except for the fluid inlet and outletports.

Other objects and advantages of the present invention will be apparentfrom the following detailed description and accompanying drawings, inwhich:

FIGURE 1 is a perspective view of an improved fluid mixing deviceembodying the present invention with portions of the outside envelopebroken away to show the internal structure more clearly;

FIG. 2 is a side elevation view of the mixing device of FIGURE 1; and

FIG. 3 is an end elevation view of the mixing device of FIGURE 1 takenfrom the inlet end.

While the invention will be described in connection with a preferredembodiment, it is to be understood that the invention is not to belimited to the disclosed embodiment but, on the contrary, it is intendedto cover the various modifications and equivalent arrangements includedwithin the spirit and scope of the appended claim.

Turning now to the drawings, the illustrative embodiment of thisinvention is made entirely of glass, although it will be apparent thatother construction materials could be utilized without departing fromthe spirit and scope of the invention. The illustrative mixing device isadapted to receive the two fluids to be mixed through an inlet port 10,with the resultant mixture being subsequently discharged from the devicethrough a downstream outlet port 12. In order to achieve somepreliminary mixing action, the two fluids are preferably fed into asingle feed line upstream of the mixing device, so that the fluidsPatented Sept. 23, 1969 to be mixed enter the inlet port 10simultaneously. The illustrative device has been used with outstandingsuccess in a high speed atmosphere monitoring instrument in whichsamples of atmospheric air are contacted with a liquid colorimetricreagent for the selective analysis of various pollutants, but it will beapparent that the device is equally useful in numerous other mixingapplications. In the atmosphere monitoring application, a short responsetime, i.e., fast development of color in the reagent, is especiallyimportant for accurate pollutant detection, and thus it is essentialthat the mixing device achieve an intimate admixture of the air andreagent rapidly and efficiently While maintaining a high fluid flowrate.

In accordance with one important aspect of this invention, the mixingdevice is adapted to pass the incoming fluids through an elongated tubeat the end of which the fluids are simultaneously passed through aprimary mixing stage comprising a cylindrical frit and a surroundingannular chamber which maintains a relatively high fluid flow rate whiledirecting the fluid mixture back along the outer surface of the tube. Ata point spaced away from the frit toward the inlet port of the tube, thefluids are passed through a secondary mixing chamber containing aparticulate packing which further advances the mixing action, with theresulting mixture being quickly discharged through the outlet port.Thus, in the illustrative embodiment, the incoming air and liquidreagent are passed through an elongated glass tube 14 which terminatesat its discharge end in a cylindrical glass frit 16. The air and reagentare passed simultaneously through the frit 16 so as to form amultiplicity of minute air bubbles within the liquid reagent therebyexposing a maximum surface area of reagent to the air sample. The degreeof mixing achieved in the primary mixing stage is determined mainly bythe pore size of the glas frit and the pressure drop across the frit. Inoperation, the pore size of the frit is fixed, but the pressure drop maybe controlled by adjusting the pressure diiferential between the inletport 10 and the outlet port 12. In a typical case where air and liquidreagent are fed into the mixing device at flow rates of one liter perminute and one milliliter per minute, respectively, a suitable pressuredrop across the frit is about six inches of water. The desired pressuredrop may be provided by conventional means, such as by feeding thefluids into the tube 14 at atmospheric pressure while maintaining theoutlet 12 of the device at a desired subatmospheric pressure.

To insure that the glass frit passes both the gas and liquid phases ofthe fluid mixture, it is preferred to use a relatively coarse frit.Extremely fine frits tend to reject the liquid phase whilepreferentially passing only the gas phase. Also, the frit must havesufficient surface area to pass the fluid mixture at the desired flowrate. The required frit pore size and surface area can be readilydetermined for any particular system in an empirical manner byconducting a few simple tests under the desired conditions. It ispreferred to use a frit which is cylindrical in shape so as to obtain360 dispersion. Moreover, if insoluble particles are accidentally drawninto the cylindrical frit, they normally affect only the dispersionthrough the end of the cylinder; dispersion through the 360 side wallsof the frit is unimpaired so that the operating efliciency of the deviceis maintained relatively high. The cylindrical frit also providesrelatively even or uniform turbulence, which is important to maintain ahigh flow rate between the two mixing stages.

In keeping with the present invention, the air-reagent mixture emergingfrom the glass frit 16 is received in an annular chamber 18 whichdirects the biphase mixture back along the outer surface of the tube 14in a direction counter to that of the incoming fluid inside the tube 14.The annular chamber 18 is formed by a glass envelope 20 whichconcentrically surrounds the tube 14 and is enlarged as at 22 to form asecondary mixing chamber 24 longitudinally spaced away from the frit 16toward the inlet end of the tube 14. Since the fluid flow issubstantially unimpeded within the annular chamber 18 interconnectingthe two mixing stages, the fluid flow rate within the mixing device ismaintained at a relatively high level, and the holdup volume isminimized. Consequently, the overall mixing time, and thus the reagentresponse time, is considerably reduced in comparison with the timeperiods required in mixing devices available heretofore. Moreover, thecountercurrent flow pattern on opposite sides of the feed tube 14 withinthe device permits an extremely compact and efiicient construction.

For the purpose of moving the fluid mixture quickly away from theoutside of the glass frit 16 and along the outside surface of the feedtube 14, the radial width of the annular chamber 18 should be as smallas possible so that there is no substantial empty volume for fluidaccumulation. Consequently, the fluid emerging from the fritcontinuously urges the preceding fluid back through the annularpassageway toward the secondary mixing chamber 24. Similarly, the end1611 of the frit should be located as close as possible to the end 2011of the glass envelope so as to avoid the possibility of forming a stalefluid pocket in the end region of the chamber 18.

-It is important that the inlet to the secondary mixing chamber 24 bespaced away from the frit 16 so that there is a substantialinterconnecting passageway 19 between the two mixing stages. Thisprovides a relatively low resistance volume for facilitating rapidremoval of fluid from the discharge side of the frit 16, and foraccelerating the fluid stream prior to its entry to the secondary mixingchamber 24.

In order to effect a secondary mixing action within the chamber 24, abed of glass beads 26 is contained within the chamber adjacent theentrance end thereof. As the biphase fluid mixture enters the chamber24, the beads 26 break and reform the air bubbles while maintaining thetwo phases in continuous intimate contact. To achieve optimum mixing,the bed of glass beads 26 preferably extends along about two thirds ofthe axial length of the chamber 24, with an open space remaining at thedischarge end of the chamber for fast fluid discharge. The diameter ofthe beads 26 should be such that the resistance offered by the bead bedto fluid flow is not substantially greater than that offered by theglass frit. A bead diameter of about three millimeters is suitable formost applications.

In addition to furthering the mixing action to provide a large interfacebetween the two fluids, the glass beads 26 serve to prevent excessivefrothing of the liquid-gas mixture. Although glass beads have beendescribed and illustrated as the preferred packing material in theillustrative embodiment of the invention, it will be understood thatother particulate packing materials that are chemically inert andheavier than the fluids involved, can be used.

In accordance with one feature of this invention, the velocity of theliquid-gas mixture is preferably increased just prior to entering thesecondary mixing stage so that the fluid mixture strikes the glass beadswith increased force and is quickly broken up. Thus, a constriction 28is formed in the glass envelope adjacent the entrance end of the chamber24 so as to reduce the cross section of the interconnecting chamber 19at that point, thereby producing a corresponding increase in the fluidvelocity. Since the velocity increase is in the axial direction, itserves to reduce the fluid flow-through time as well as enhancing themixing action. The constriction 28 also serves to prevent the glassbeads from becoming accidentally lodged in the annular passageway 19.

For the purpose of discharging the fluid mixture from the secondarymixing chamber, a transfer tube 30 is provided near the downstream endof the chamber 24. The intake port for the transfer tube 30 is locatedin the open space beyond the downstream side of the bed of glass beads26 so that the fluid mixture is continuously discharged at a high flowrate. With the mixing device provided by this invention, conditions ofhigh velocity and turbulence are achieved in the transfer tube tofurther assure good mixture of the two phases.

In order to prevent the glass beads 26 from being swept out of thesecondary mixing chamber 24 into the transfer tube 30, a constriction 32is provided at the inlet end of the tube 30 so as to reduce the intakeport to a size smaller than that of the glass beads. Consequently, thereis no chance of the beads clogging the transfer tube or any otherdownstream portion of the instrument in which the mixing device isutilized. Moreover, in addition to trapping the beads within the chamber24, the constriction 32 serves to increase the velocity of the fluidstream as it enters the transfer tube 30, thereby enhancing theturbulent mixing conditions within the transfer tube.

Although the mixing device of this invention has been described withparticular reference to the mixing of only two fluids, it will beunderstood that the invention is equally applicable to the mixing ofpractically any number of fluids. In the exemplary air-reagentapplication described above, a number of different liquid reagents maybe employed, but a particularly preferred reagent for detecting nitrogendioxide in air samples is the reagent described in copending applicationSer. No. 462,226, now issued as Patent No. 3,375,079 entitledColorimetric Reagent for Analysis of Nitrogen Dioxide in Air. The

reagent described in that application has an extremely.

fast response time and, therefore, is especially compatible with thefast flow rates provided by the device of the present invention.

As can be seen from the foregoing detailed description, the improvedmixing device provided by this invention is capable of rapidly achievingan intimate admixture of two or more fluids while maintaining a highflow rate of the fluids being mixed. Actual fluid tests havedemonstrated that the mixing device of this invention is especiallysuitable for use in contacting a liquid colorimetric reagent with a gassample in a high speed colorimetric gas analyzer, and that the mixingdevice provides a substantially shorter response time for thecolorimetric reagent than any other mixing device heretofore. Moreover,the device of this invention provides a minimum volume holdup of thefluids being mixed while maintaining a continuous and rapid fluid flowduring the mixing operation. Furthermore, the improved mixing deviceprovided by this invention is extremely compact so that it is compatiblefor use in small portable gas analyzers. The inventive mixing device isalso simple and economical to manufacture and maintain, and iscompletely enclosed except for the fluid inlet and outlet ports.

I claim as my invention:

1. Apparatus for rapidly mixing two or more fluids, said apparatuscomprising the combination of a primary mixing stage including a fritfor breaking up the two fluids so as to provide a large interfacebetween the two fluids, an elongated tubular member for feeding the twofluids simultaneously to said frit, an annular chamber surrounding saidfrit and a substantial portion of said tubular member for receiving thefluid mixture emerging from the frit and directing the fluid mixtureemerging outer surface of the tubular member in a direction counter tothat of the incoming fluids inside the tubular member, and a secondarymixing stage surrounding said References Cited tubular member forreceiving the fluid mixture from UNITED STATES PATENTS said annularchamber, said secondary mixing stage being spaced away from said frittoward the inlet end of said 1,887,126 11/1932 Grossman 23-292 tubularmember and including a bed of particulate ma- 2382381 8/1945 Calvert etterial for again breaking up the two fluids to provide a 0 2,394,0232/1946 smckland large interface between the two fluids, and an outletport for discharging the resultant fluid mixture, said annular JAMESTAYMAN Pnmary Exammer chamber including a restriction adjacent saidsecondary U S C1 XR mixing stage to increase the velocity of the fluidmixture 10 entering said secondary mixing stage. 23254; 2594; 261-94, 96

1. APPARATUS FOR RAPIDLY MIXING TWO OR MORE FLUIDS, SAID APPARATUSCOMPRISING THE COMBINATION OF A PRIMARY MIXING STAGE INCLUDING A FRITFOR BREAKING UP THE TWO FLUIDS SO AS TO PROVIDE A LARGE INTERFACEBETWEEN THE TWO FLUIDS, AN ELONGATED TUBULAR MEMBER FOR FEEDING THE TWOFLUIDS SIMULTANEOUSLY TO SAID FRIT, AN ANNULAR CHAMBER SURROUNDING SAIDFRIT AND A SUBSTANTIAL PORTION OF SAID TUBULAR MEMBER FOR RECEIVING THEFLUID MIXTURE EMERGING FROM THE FRIT AND DIRECTING THE FLUID MIXTUREEMERGING OUTER SURFACE OF THE TUBULAR MEMBER IN A DIRECTION COUNTER TOTHAT OF THE INCOMING FLUIDS INSIDE THE TUBULAR MEMBER, AND A SECONDARYMIXING STAGE SURROUNDING SAID TUBULAR MEMBER FOR RECEIVING THE FLUIDMIXTURE FROM SAID ANNULAR CHAMBER, SAID SECONDARY MIXING STAGE BEINGSPACED AWAY FROM SAID FRIT TOWARD THE INLET END OF SAID TUBULAR MEMBERAND INCLUDING A BED OF PARTICULATE MATERIAL FOR AGAIN BREAKING UP THETWO FLUIDS TO PROVIDE A LARGE INTERFACE BETWEEN THE TWO FLUIDS, AND ANOUTLET PORT FOR DISCHARGING THE RESULTANT FLUID MIXTURE, SAID ANNULARCHAMBER INCLUDING A RESTRICTION ADJACENT SAID SECONDARY MIXING STAGE TOINCREASE THE VELOCITY OF THE FLUID MIXTURE ENTERING SAID SECONDARYMIXING STAGE.